Continuous process for digesting cellulosic material



Aug. 1o, 1965 J. C. F. C. RICHTER ETAL CONTINUOUS PROCESS FOR DIGESTING CELLULOSIC MATERIAL Filed Dec. 17, 1962 7;) foune I@ ffrarfer ATTORNEYS United States Patent O 3,200,032 CNTINUQUS PROCESS FR DEGESTHNG CELLULOSIC MATERAL Johan C. F. C. Richter, Alpes-Maritimes, France, and

@le Il. Richter, Karlstad, Sweden, assignors to Aktieiroiaget Kamyr, Karlstad, Sweden, a company of Sweden Filed Dec. 17, 1962, Ser. No. 245,2ti2 maints priority, application Japan, Dec. 23, 1961, S56/46,936 5 Claims. (C1. 162-19) The present invention relates to a continuous two-stage process for digesting cellulose and appartus adapted for carrying out this process.

The principal object of the invention is to provide a new and highly advantageous process and apparatus for the two-stage continuous digestion of cellulosic fiber material. A more speciiic object of the invention is the provision of a two-stage cellulosic disgesting process and apparatus which are more economical and effective than prior proccdures and devices. Other objects will also be hereinafter apparent.

Broadly stated, the process of the invention involves charging cellulosic fiber material into one end of an elongated disgester, subjecting this material to a digesting cool: in two successive stages where the digesting liquor in the first stage flows concurrently with the tiber materiais and the liquor in the second stage flows countercurrently, discharging the thus digested material from the opposite end of the digester, washing the digested material subsequent to the second digesting cooi; but before discharge from the digester, and filtering od the spent digesting liquors from the digester in a Zone intermediate the inst and second digesting stages.

The apparatus contemplated herein comprises an elongated digester having a material inlet end and a material discharge end, means for continuously charging iiber material into the inlet end of the digester, means for continuously discharging digested fiber material from the discharge end of the digester, two successive digesting .zones positioned between the inlet and discharge ends of the digester, means for passing digesting liquor concurrently with the material through the first digesting zone, means for passing digesting liquor couutercurrently to the material through the second digesting zone, and sieve means intermediate the two digesting zones for straining ott spent digesting liquors from the digester.

The success of the invention is due to the combination of several unique features including concurrent flow between the digesting liquor and cellulosic iiber material in the first stage, countercurrent flow between the liquor and iiber material in the second stage and straining oit spent digesting liquor from the iber material in an intermediate or bordering zone between the digesting stages.

rthe straining operation may be performed through separate sieves, one for each digesting liquor, located relatively close to each other, or at comparatively small mutual distances.

Washing of the digested fiber material is preferably performed by passing a washing liquid comprising water, or tiltrate with digesting liquor residues, in countercurrent flow through the fiber material, after the second digesting stage but before discharge of the material from the digester. To this end, the wash liquid is desirably passed into the digester, at or near the discharge end thereof, through a washing area to the second digestlidddfiz Patented Aug. 10, 1965 ing stage, where it is at least partly mixed with the digesting liquor of the second stage. The resulting mixture of digesting liquor and washing liquid is then passed through the second digesting stage in countercurrent ow with the liber material.

in order to accomplish the opposing flow patterns required herein, the elongated digester of the invention is provided with appropriate sieve means, specifically, at least one, and generally, two sieve girdles, which are adapted to strain oit spent digesting liquors from the iiber material and are arranged along the wall of the digester at about Mi to 1/2 the length thereof, measured from the material inlet end. The digester is also provided with conduit means at opposite sides of the sieve means for supplying two diterent digesting liquors so that the digesting liquors will pass through the ber material in opposite directions, i.e., first in concurrent ilow and then countercurrently, with respect to the fiber materal. The spent liquors are then discharged through the sieve girdles in the intermediate zone between the two digesting stages.

The invention is more fully explained with reference to the accompanying drawing which diagrammatically illustrates a preferred embodiment of the digesting plant and process according to the invention.

Referring to the drawing, the numeral 11 designates an upright cylindrical digester of essentially uniform crosssectional area, and a length equal to about ten times the diameter. At its upper end, the digester is provided with appropriate charging or inlet means 13 of a type well known in the art, for example, as shown in United States Patent No. 2,459,180, utilizing a screw conveyor in the charging device, which is shown in more detail in Patent No. 2,459,180. A comminuted cellulosic material, such as wood chips, is charged by the means 13 into the digester continuously or in the form of discrete successive batches. The thus charged material moves aS a compact column vertically downward through the digester and is discharged at its lower end into a discharge conduit, generally indicated by the numeral 15, by means of a suitable discharging device, such as that shown in United States Patent No. 2,938,824. The discharging device, indicated generally as the pulp outlet 59, utilizes a screw conveyor which is shown in more detail in Patent No. 2,938,824. The introduction and direction of feed of the charge material is indicated by the double arrows 17. During its passage through the digester, the fiber material is subjected to various treatment areas which are indicated as the separate Zones a, b, c and d, respectively.

in the first comparatively short zone a, impregnation of the wood chips or cellulosic ber material takes place by means of the concurrent introduction of the digesting liquor together with the chips, as indicated by liquor feed arrow 19. The digesting liquor of the first digesting stage preferably consists of sulphate lye, c g., sodium or calcium sulphate lye. As the arrows 21 indicate, the digesting liquor in zone a tiows in the same direction as the fiber material. When the mixture of material and digesting liquor reaches the level of a sieve girdle 23, which is inserted into the wall of the digester, the mixture is heated. To this end, digesting liquor is sucked out through sieve girdle 23 by means of pump 2S, re-circulated in outer conduit 26 and through heating device 27, e.g., a heat exchanger, and then re-introduced through return tube 29, the latter terminating on the axis of the digester and discharging at approximately the level of, or somewhat above the level of, the sieve girdle 23. The liquor circulating in said loop is heated by steam supplied as shown at fili to heat exchanger 27 to the desired temperature (eg, i0() to l75 C). The fiow of liquor in the described circulation loop may be to 10 times as large as the flow of liquor in the longitudinal direction of the digester.

The heated liquor spreads from the orifice of the tube 29 radially outwards and heats the charge material to the desired digesting temperature. As an aiternative, the

heating may also take place by directly injecting steam into the outer circulation conduit or loop 26 or directly into `the digester.

The first stage digestion takes place in zone b, so that when the fiber material reaches the level of the second 'sieve girdle 33, digestion with the lye liquor is completed,

and the lye is essentially spent. The lye liquor is strained olf through sieve girdle 33, and a part thereof is pumped by means of pump 35 back into circulation loop 25, and

thus is returned to the digester through the orifice of the conduit 29. Another part of the liquor will be carried off through conduit 37 for recovery of its heat and/or chemical content.

As the arrows 39 indicate, the lye liquor in zone b still fiows in the same direction as the fiber material and by recirculation of the lye liquor, the liquor will move at a speed higher than the speed of the fiber charge material.

'Hence the lye liquor exerts a downwardly directed pressure kupon the material, which pressure, together with the gravity acting thereupon, aids in feeding the material coltotal length of the digester, and preferably at a point about 1/3 of the length thereof. Sieve girdle 41 serves to strain off from the fiber material the spent digesting liquor employed in the second digesting stage c which digesting liquor flows upwardly in countercurrent relation to the direction of the fiber material in this stage, as indicated by the arrows 43. Between the two opposing liquor currents 39 and 46, there is formed a transition zone where the liquid is substantially stagnant or immobile.

Digesting liquor for the second stage is supplied through conduit 53 to pump 5l and then into digestion column lf by means of heat exchanger t7 and outlet orifice 4&9 at sieve girdle 45. The second digesting zone c extends from sieve girdle 41 to the fourth sieve girdle 45, the latter being situated at a distance from the discharge end of the digester corresponding approximately to 1/s of the length of the digester. Sieve girdle i5 is connected into a circulation loop 48 including heating device 47 and return tube 49, located centrally in the digester and having an outlet orifice located at the level of the sieve girdle d5, or somewhat higher. Part of the second digestion liquor is recovered at sieve girdle 45 and re-circulated through conduit 48 for reheating as in the case of the first digesting stage.

'completely digested fiber material is performed by means of washing liquid. This washing liquid may consist of pure water, or of filtrate taken from a subsequent wash filter and containing digesting liquor residues. The washing liquid is supplied through spray nozzles S7 distributed around the periphery of the digester near pulp outlet 59. A part of the wash liquid, which also has a cooling effect, accompanies the pulp out through outlet 59. Another part of the wash liquor is forced upwards through the material column and countercurrent thereto, as indicated by the arrows of, thereby displacing the digesting liquor from the orice of tube 49, and forcing the same to move upwardly in countercurrent flow with respect to the fiber material, see arrows 43. At the level of sieves 33 and 41, the second stage digesting liquor, which is substantially spent, encounters the downwardly directed fluid current (see arrows 39) and is forced thereby to escape through sieve girdle 41. From there the wash liquor fraction and second stage liquor fraction are passed to a container 63, cg., in the shape of a cyclone, which is maintained at a pressure considerably lower than the digesting pressure. Liberated steam and gas are discharged from container 63 through conduit 65 in order to recover the heat content thereof, and spent liquor is discharged through conduit 67 to a chemical recovery plant.

in the case where the second stage digestion is performed with SO2, supplied through the conduit 53 as gas or as a mixture of SO2 and water, the SO2 is let off through conduit 65 for return tothe digester.

It is to be understood that the present invention is not limited to the digesting chemicals used herein but can be applied equally well with other digesting chemicals and processes which are susceptible and amenable to two-stage continuous operations in a single digester or vessel. To permit such use of the present invention, it is only necessary that the same pressure be used in the two treating stages, whereas the temperatures can be varied since it is Apossible for the temperature of the first stage herein to be considerably higher or lower than that of the second stage.

As will be appreciated, the present invention provides for the separation of spent lyes from distinct stages to permit recovery of their separate chemical contents. If recovery is not essential, or if the two types of spent lyes can be recovered simultaneously after mixing them, the conduits from sieves 33 and 41 can be connected together by means of conduit 69, whereby the recovery of both heat and chemical is greatly simplified.

The present invention may also be used in those cases where it is desired to employ the same digesting liquor in the two stages but where the stages differ in other respects, such as temperature and concentrations. Thus, many changes may be made in the foregoing description without departing from the inventive scope thereof as defined in the following claims wherein:

We claim:

i. In a continuous process for digesting cellulosic iibrous material including the step of continuously charging cellulosic fibrous material into one end of an elongated digester; impregnating the cellulosic fibrous material with digesting liquor adjacent said one end `of the digester; digesting the impregnated material by treating the material with heated digesting liquor while passing it through the central portion of the digester; washing the material adjacent the other end of the digester; and discharging the digested material from said other end of the digester; the improvement which comprises conducting the digesting step in two stages by digesting the cellulosic fibrous material in a first digesting Zone in the central portion of the digester with heated digesting liquor owing through the first digesting zone concurrently with the cellulosic fibrous material; digesting the cellulosic fibrous material in a second digesting zone in the central portion of the digester with `a different digesting liquor flowing through the second digesting zone countercurrently with the cellulosic fibrous material; separtely straining oft the respective spent digesting liquors from the two digesting zones in an intermediate zone therebetween, the straining being conducted simultaneously and continuously in closely spaced separating areas within the intermediate zone; washing the digested cellulosic fibrous material following the second digesting step with wash liquid fiowing countercurrently with the celiulosic fibrous material, the wash liquid being strained off together with second stage digesting liquor; and thereafter discharging the digested cellulosic fibrous material.

2L. The process of claim 1 wherein the liquor inthe first digesting stage is sulphate digesting liquor and the liquor in the liquor in the second digesting stage is sulphur dioxide digesting liquor.

3. The process of claim 1 wherein said wash liquid is passed countercurrently to said material from a point adjacent the discharge end of said digester, said wash liquid is at least partly mixed with the diilerent digesting liquor passing into said second digesting stage and the resulting mixture is passed through said second stage in countercur-rent to said material.

4. The process of claim 1 wherein a part of the digesting liquor is Withdrawn from each digesting stages, heated and returned to said stage for further digesting.

5. The process of claim 1 wherein said material is Washed after said second digesting zone and filtrate `consisting of water and residues of digesting liquor previously (i discharged from said digester together with the fiber material, said filtrate owing countercurrently lo said mate-v rial.

References Cited by the Examiner' UNITED STATES PATENTS 2,695,232 ll/54 Richter 162--237 2,920,697 1/6() Langen 162--237 3,035,963 5/62 Schnyder 162-19 3,097,987 7/63 Sloman 162-17 OTHER REFERENCES Casey, Pulp and Paper, vol. 1, 2nd edition, 1960, Interscience Publishers Inc., New York, page 179.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. TWOLK, Examiner. 

1. IN A CONTINUOUS PROCESS FOR DIGESTING CELLULOSIC FIBROUS MATERIAL INCLUDING THE STEP OF: CONTINUOUSLY CHARGING CELLULOSIC FIBROUS MATERIAL INTO ONE END OF AN ELONGATED DIGESTER; IMPREGNATING THE CELLULOSIC FIBROUS MATERIAL WITH DIGESTING LIQUOR ADJACENT SAID ONE END OF THE DIGESTER, DIGESTING THE IMPREGNATED MATERIAL BY TREATING THE MATERIAL WITH HEATED DIGESTING LIQUOR WHILE PASSING IT THROUGH THE CENTRAL PORTION OF THE DIGESTER; WASHING THE MATERIAL ADJACENT THE OTHER END OF THE DIGESTER; AND DISCHARGING THE DIGESTED MATERIAL FROM SAID OTHER END OF THE DIGESTER; THE IMPROVEMENT WHICH COMPRISES CONDUCTING THE DIGESTING STEP IN TWO STAGES BY DIGESTING THE CELLULOSIC FIBROUS MATERIAL IN A FIRST DIGESTING ZONE IN THE CENTRAL PORTION OF THE DIGESTER WITH HEATED DIGESTING LIQUOR FLOWING THROUGH THE FIRST DIGESTING ZONE CONCURRENTLY WITH THE CELLULOSIC FIBROUS MATERIAL; DIGESTING THE CELLULOSIC FIBROUS MATERIAL IN A SECOND DIGESTING ZONE IN THE CENTRAL PORTION OF THE DIGESTER WITH A DIFFERENT DIGESTING LIQUOR FLOWING THROUGH THE SECOND DIGESTING ZONE COUNTERCURRENTLY WITH THE CELLULOSIC FIBROUS MATERIAL; SEPARATELY STRAINING OFF THE RESPECTIVE SPENT DIGESTING LIQUORS FROM THE TWO DIGESTING ZONES IN AN INTERMEDIATE ZONE THEREBETWEEN, THE STRAINING BEING CONDUCTED SIMULTANEOUSLY AND CONTINUOUSLY IN CLOSELY SPACED SEPARATING AREAS WITHIN THE INTERMEDIATE ZONE; WASHING THE DIGESTED CELLULOSIC FIBROUS MATERIAL FLOWING THE SECOND DIGESTING STEP WITH WASH LIQUD FLOWING COUNTERCURRENTLY WITH THE CELLULOSIC FIBROUS MATERIAL, THE WASH LIQUID BEING STRAINED OFF TOGETHER WITH SECOND STAGE DIGESTING LIQUOR, AND THEREAFTER DISCHARGING THE CELLULOSIC FIBROUS MATERIAL. 